Information About CDP

Cisco Discovery Protocol (CDP) runs over the data link layer and is used to advertise information to all attached Cisco devices, and to discover and view information about attached Cisco devices. CDP runs on all Cisco-manufactured equipment.

CDP gathers protocol addresses of neighboring devices and discovers the platform of those devices. CDP runs over the data link layer only. Two systems that support different Layer 3 protocols can learn about each other.

Each device you configure for CDP sends periodic advertisements to a multicast address. Each device advertises at least one address at which it can receive SNMP messages. The advertisements also contain hold-time information, which indicates the length of time that a receiving device should hold CDP information before discarding it. You can configure the advertisement or refresh timer and the hold timer.

CDP Version 2 (CDPv2) allows you to track instances where the native VLAN ID or port duplex states do not match between connecting devices.

CDP advertises the following type-length-value fields (TLVs):

•Device ID

•Address

•Port ID

•Capabilities

•Version

•Platform

•Native VLAN

•Full/Half Duplex

•MTU

•SysName

•SysObjectID

•Management Address

•Physical Location

All CDP packets include a VLAN ID. The CDP packet is untagged, so it goes over the native/access VLAN, which is then also added to the packet.